A conventional photovoltaic cell structure is fashioned by bringing together n-type and p-type semiconductors to form a p-n junction. A negative electrode is typically located on the side of the cell that is to be exposed to a light source (the “front” side, which in the case of a solar cell is the side exposed to sunlight), and a positive electrode is located on the other side of the cell (the “back” side). Radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate electron-hole pairs in that body. Because of the potential difference that exists at a p-n junction, holes and electrons move across the junction in opposite directions, giving rise to the flow of an electric current that is capable of delivering power to an external circuit. Most industrial photovoltaic cells, including solar cells, are provided in the form of a structure, such as one based on a doped crystalline silicon wafer, that has been metallized, i.e., provided with electrodes in the form of electrically conductive metal contacts through which the generated current can flow to the external electric circuit load.
Photovoltaic cells are commonly fabricated with a front-side insulating layer that affords an antireflective property to the cell to maximize the utilization of incident light. However, in this configuration, the insulating layer normally must be removed to allow an overlaid front-side electrode to make contact with the underlying semiconductor surface. The front-side electrode is typically formed by first depositing a metal-powder-bearing, conductive paste composition in a suitable pattern by screen printing. Thereafter, the paste is fired to dissolve or otherwise penetrate the insulating layer and sinter the metal powder, such that an electrical connection with the semiconductor is formed.
The ability of the paste composition to penetrate the antireflective coating and form a strong bond with the substrate upon firing is highly dependent on the composition of the conductive paste and firing conditions. Efficiency, a key measure of photovoltaic cell performance, is also influenced by the quality of the electrical contact made between the fired conductive paste and the substrate.
Allison et al. (U.S. Pat. Nos. 5,089,172 and 5,393,558) disclose a thick-film conductor composition that can be bonded to a ceramic substrate fashioned from aluminum nitride.
Although various methods and compositions useful in forming devices such as photovoltaic cells are known, there nevertheless remains a need for compositions that permit fabrication of patterned conductive structures that result in improved overall device electrical performance and that facilitate the efficient manufacture of such devices. A lead-free composition would be particularly desirable.